The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Young smokers turning to e-cigarettes

E-cigarettes are becoming increasingly popular among young adults in NSW, with many believing they aren't as harmful as traditional cigarettes despite a lack of medical evidence.

New research shows that 18-29 year olds are the biggest users of the battery-powered devices, but smoke them less frequently than older adults.

While adults over 30 are most likely to use e-cigarettes to help them quit or cut down smoking, a quarter of young people use them because they are "not as bad for your health" as cigarettes.

The findings were based on a Cancer Institute of NSW study, which surveyed about 3000 tobacco smokers and people who recently quit to find out who used e-cigarettes and why.

E-cigarettes are legal in Australia but the nicotine in them is classified as a dangerous poison, the sale and possession of which is illegal.

Debate has been raging about the safety of e-cigarettes, with Britain's Royal College of Physicians in April backing their promotion as a safe alternative to smoking.

But critics argue that there is little known about the long-term health effects of the devices, which produce a vapour inhaled by the user, and claim they encourage people to smoke tobacco.

Nine per cent of those surveyed by the Cancer Institute said they had used e-cigarettes in the 18 months to June 2015, up from seven per cent in 2013.

The rate of e-cigarette use was highest (16 per cent) among those aged 18-29, with most also still smoking tobacco.

While the most popular reason among young people for using e-cigarettes was health, many also believed they would help them quit.

Experts from the Cancer Institute said they were worried that so many people were using e-cigarettes as well as traditional cigarettes.

"Despite the fact that many e-cigarette users reported using them to assist quitting, most users in this sample were dual users," the study's lead author, Sally Dunlop, wrote in the Medical Journal of Australia on Monday.

"Many reported that they used e-cigarettes to cut down on the number of cigarettes they smoked. For avoiding the risks of smoking-related premature death, however, reducing cigarette numbers is much less effective than quitting."

Despite a ban on the sale of e-cigarettes containing nicotine in Australia, most users are buying them from overseas via the internet.

In an editorial published in the MJA, experts from the London School of Hygiene and Tropical Medicine, Curtin University and Sydney University argued that regulators need to take a twin-track approach to e-cigarettes, with tighter rules around their marketing and sale.

Controls applied to tobacco product promotions should also be applied.